As is known in the art, carbon nanotubes (CNT) materials are light weight electrical and thermal conductors. Commercially available CNT materials, such as for example shown in FIG. 1, comprise a plurality of carbon nanotubes (CNTs) bonded together in the form of sheet, tape, and yarns could be utilized in more electrical applications if its conductivity was more like copper (5.9E+7 S/m). Higher electrical conductivity CNT materials would enable lightweight, flexible circuit structures, key to a variety of buoyant and space radar platforms where meeting weight restrictions are critical. Higher thermal conductivity CNT materials would enable lightweight, flexible heat dissipating structures. Presently, CNT materials have electrical conductivity 1/10 of copper at high frequencies and 1/25 the conductivity at or near DC.
As is also known in the art, attempts have been made to increase electrical conductivity which can be characterized as mechanically and dopant based. Efforts are being made to increase the length of the CNTs, align the CNTs more parallel and end-to-end, and to densify the sheet and yarn type structure of carbon nanotubes materials by multi-layer compression. Researchers have also been investigating prudent doping of the materials. The addition of selected dopants has shown significantly higher conductivity. Most examples of doping that have shown significant improvement rely on the introduction of large atoms where a more diffuse and extended electron orbitals can promote electron connective paths between CNTs as well as form intermediate states in the semi-conductor CNT bandgaps increasing electronic conductivity. In most cases, the atoms are diffused into place. In a few situations, expensive transition metal doping (i.e. Pt, Pd) has been used. Such atoms have shown coordination affinity to the CNTs. The inventors have recognized that since the dopants are not chemically bonded in place, the aforementioned Van der Waal affinity makes than subject to property instability upon applied stress. Due to the relatively weak forces involved in Van der Waal bonding, distortion of individual CNTs and relative motion of adjacent CNTs can cause the dopant atoms to become dislodged from their original locations leading to change in properties.
In accordance with the present disclosure, a structure is provided comprising: a carbon nanotube material comprising a plurality of carbon nanotubes; and an electrically or thermally conductive material disposed on at least a portion of the carbon nanotubes, such material being chemically bonded to such portion of the carbon nanotubes.
In one embodiment, the electrically or thermally conductive material is covalently (i.e., chemically) bonded to such portion of the carbon nanotubes.
In one embodiment, molecules of the electrically or thermally conductive material are bonded to a plurality of the portion of the carbon nanotubes,
In one embodiment, the electrically or thermally conductive material is a conductive polymer forming conjugated electron bridges between a plurality of the carbon nanotubes.
In one embodiment, a source of the electrically or thermally conductive material comprises polyacetylene.
In one embodiment, a source of the electrically or thermally conductive material comprises polyphenylacetylene.
In one embodiment the polymer is doped.
In one embodiment, the dopant is iodine or bromine.
In one embodiment, a method is provided comprising: providing a carbon nanotube material comprising a plurality of carbon nanotubes; providing a source of conductive monomer; transporting the monomer in gas form to the carbon nanotube material; converting the monomer to a conductive polymer; and chemically bonding the conductive polymer to the plurality of carbon nanotubes.
In one embodiment, the monomer is acetylene.
In one embodiment, the monomer is gaseous.
In one embodiment, the monomer is blended with gas.
In one embodiment, the gas is nitrogen or helium.
In one embodiment, the monomer is phenylacetylene.
In one embodiment, the transporting comprises passing a gas through a liquid monomer.
In one embodiment, the gas is nitrogen.
In one embodiment, the transporting comprises vaporizing the monomer.
In one embodiment, the vapor comprises bubbling the gas through the liquid monomer.
In one embodiment, the chemical bonding comprises using disassociation by thermal or photon mediated means.
In one embodiment, the photolysis comprises using UV photons.
In one embodiment, the photolysis comprises using a xenon arc lamp.
In one embodiment the carbon nanotube material is heated by passing current through it, thereby accomplishing thermal disassociation of monomer units.
In one embodiment, the monomer is transported to the carbon nanotube material in gas phase by flow either around or through CNT material.
In one embodiment, a polymerization process is used to chemically bond the conductive polymer onto the carbon nanotubes comprising: using ultraviolet (UV) photon energy optimized for particular molecular species in combination with an elevated temperature CNT material.
In one embodiment, the elevated temperature is used only on the nanotubes.
In one embodiment, phased intermittent flow of monomer, UV energy, and thermal energy are used for optimal placement of conductive bridges between pairs of CNT materials.
In one embodiment, the polymer is grown on selected regions of the nanotubes.
In one embodiment, the polymer is grown on junctions between pairs of the carbon nanotubes.
In one embodiment, the polymer is grown on resistive sites between the carbon nanotubes.
With such structure and method, the inventors have recognized that although electron travel within a CNT has little resistance, the finite length of each CNT results in a structure in which electrons have to move from CNT to CNT at the various crossing points (junctions) between the CNTs in order to get across the bulk material and that these junctions contribute a significant amount to the overall resistance of the CNT. More stable and higher electrical conductivity can be obtained by chemically modifying the CNT material so that gaps between the CNTs are bridged by conjugated pi-bond molecular chains that are bonded to or overlap the CNT ends or the CNT walls. With sufficient production of CNT to CNT bridges, electron travel will be facilitated thus reducing the inherent resistance and increasing the conductivity to be more metal-like. It should be noted that due to the fact that a portion of thermal conductivity of a material is mediated by electron transport, any improvements to the electrical conductivity will simultaneously result in an improvement in thermal conductivity as well. Commercially available CNT material has been exposed to non-optimized prescribed set of energetic and chemical conditions which resulted in an increase of electrical conductivity of 20%. This electro-physical effect will provide a tool for continued improvement of the CNT material to be metal-like.
Further, 1) The electrical conductivity increase of the formed CNT material can be obtained by photo-thermal energetic exposure of unsaturated, conjugated molecules in a flow reactor, 2) Conjugated pi-bond chains can be formed between CNTs to promote defined conductive paths for electron travel; 3) Defined electron paths between CNTs are chemically bonded in place which simultaneously makes-them resistant to flex, diffusion or distortion; 4) Conjugated electron bridges connect CNTs in the horizontal and vertical plane; 5) The resultant material is single phase and not prone to corrosion processes by removing galvanic dissimilar materials; 6) Post-process is amenable to high volume manufacturing; and 7) Uses an inexpensive feedstock.
Still further, the method and structure: Provides more defined conductive electron paths throughout the CNT material for enhanced electron mobility and increased electrical conductivity between tubes; Conductive paths are chemically bonded in place and not prone to local stresses as observed in doping; Provides natural layer to layer randomization so that skin effect is mitigated at higher frequencies; Also, chemically bonded conjugated electron bridges between tubes and at tube crossing points facilitates electron mobility through CNT bulk material.
Still further, the process: Uses conjugated hydrocarbon in a photo-thermal energetic exposure; Forming defined electrical bridges between discrete CNTs; Stable single phase material structure; Process amenable to high volume manufacturing; Reduces skin effect at high frequencies (>THz).
Yet still further, the Polymerization Process:                Uses solid or liquid monomer applied via solution exposure which subsequently become conductive with subsequent application of energy for chemical reaction;        Uses photon energy bands such as ultra-violet selected for particular molecular species in combination with elevated temperature CNT material;        Uses elevated temperature CNT material only;        Uses intermittent material flow optimally phased with applied energy for optimal placement of conductive bridges;        Makes use of flow of monomer through the thickness of the CNT material and energy application for more effective placement of conductive bridges through sheet thickness.        
The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.